1. Field of the Invention
The present invention relates to an electrolyte, a production process therefor, an electrolyte membrane, a production process therefor, a catalyst layer and a fuel cell, and more particularly, to an electrolyte that is capable of being used in an electrolyte membrane and an electrolyte in a catalyst layer that are used in various electrochemical devices such as fuel cells, a production process therefor, and an electrolyte membrane, a production process therefor, a catalyst layer and a fuel cell using the electrolyte.
2. Description of the Related Art
A solid polymer fuel cell uses a membrane electrode assembly (MEA) in which electrodes are attached to both sides of a solid polymer electrolyte membrane as a basic unit. In addition, in the solid polymer fuel cell, an electrode generally has a two layer structure of a diffusion layer and a catalyst layer. The diffusion layer is a layer to provide a reaction gas and electrons to the catalyst layer, and carbon paper, carbon cloth and the like may be used for the diffusion layer. Additionally, the catalyst layer is a portion of a reaction field of electrode reaction and generally includes a complex of carbon carrying an electrode catalyst such as platinum and the like and a solid polymer electrolyte (electrolyte in the catalyst layer).
As the electrolyte membrane or the electrolyte in the catalyst layer that constitutes MEA, in general, a fluorocarbon-type electrolyte having excellent oxidation resistance (for example, Nafion (registered trademark, manufactured by DuPont Co., Ltd.), Aciplex (registered trademark, Asahi Chemical Co., Ltd.), and Flemion (registered trademark, Asahi Glass Co., Ltd.)) is used. Furthermore, the fluorocarbon-type electrolyte has excellent oxidation resistance but is costly. Accordingly, in order to reduce the cost of the solid polymer fuel cell, the use of a hydrocarbon-type electrolyte has been studied.
However, in order to use the solid polymer fuel cell as a power source for vehicles, some problems to be resolved have been left. For example, in order to obtain high performance in the solid polymer fuel cell, it is preferable that an operation temperature of a cell is high, and that heat resistance of the electrolyte membrane is high. However, the known fluorinated electrolyte membrane has a problem in that mechanical strength is low at high temperatures.
In addition, currently, operation of the fuel cell at high temperatures and low humidity or no humidity condition is considered to be important. Under the condition, in order to realize high proton conductivity, an electrolyte having high ion exchange capacity is required. However, since the electrolyte having the high ion exchange capacity has high water content, a swelling change of the membrane is significant. Therefore, the shape of the membrane may not be maintained or it may be dissolved in water. The trade-off of the high conductivity and the high water content is considered an important problem to be overcome.
Therefore, in order to solve the problems, some suggestions have been given.
For example, Patent Document 1 discloses a copolymer that is obtained by copolymerizing F—C6H4—SO2NKSO2—(CF2) 4—SO2NKSO2—C6H4—F, Cl—C6H4—SO2—C6H4—Cl, and HO—C6H4—C6H4—OH.
The Document describes that the conductivity of the copolymer obtained by using the above method is 156 mS/cm at the temperature of 85° C. and relative humidity of 95%.
Further, Patent Document 2 discloses a block copolymer in which a hydrophilic segment includes poly(parapenylene) as a main chain and a sulfonic acid group bonded through an alkyl group thereto as a side chain, and a hydrophobic segment includes polyether ketone.
Further, Patent Document 3 discloses a block copolymer in which a hydrophilic segment includes a vinyl polymer such as poly(α-methylstyrene) and a hydrophobic segment includes an aliphatic polymer such as polybutadiene.
Further, Patent Document 4 discloses a block copolymer in which a hydrophilic segment includes a fluorinated poly(vinyl ether) having a sulfonic acid group and a hydrophobic segment includes a fluorinated poly(vinyl ether) having no sulfonic acid group.
Further, Patent Document 5 discloses a block copolymer in which a hydrophilic segment includes a fluorinated poly(vinyl ether) having a sulfonic acid group and a hydrophobic segment includes poly ether sulfone partially having a fluorine atom.    [Patent Document 1] Japanese Patent Application Laid-Open No. 2004-331972    [Patent Document 2] Japanese Patent Application Laid-Open No. 2006-252813    [Patent Document 3] Japanese Patent Application Laid-Open No. 2006-210326    [Patent Document 4] Japanese Patent Application Laid-Open No. H11 (1999)-329062    [Patent Document 5] Japanese Patent Application Laid-Open No. 2004-190003
If a polymer being a hydrophilic segment and including high ion exchange capacity is bonded to a polymer being a hydrophobic segment to produce a block copolymer, swelling change of the membrane and dissolution in water may be suppressed. The problems may be solved by separating abilities of providing proton conductivity and mechanical strength of the membrane to the hydrophilic segment and the hydrophobic segment, respectively.
However, if the hydrocarbon polymer (it may partially include a fluorine atom) is used in both the hydrophilic segment and the hydrophobic segment of an electrolyte, the electrolyte is oxidized with hydrogen peroxide (or hydroxy radical) formed at power generation of a fuel cell and thereby a function required in the electrolyte is removed. Meanwhile, if the fluorinated polymer is used in both the hydrophilic segment and the hydrophobic segment, the oxidation resistance is good. However, if the fluorinated polymer is used in both segments, there are problems in that the cost is increased and a load to environment is increased.
In addition, in order to use the electrolyte as the electrolyte membrane for the fuel cell, not only the high heat resistance, but also high gas barrier property and predetermined flexibility are required.